In a principal aspect, the present invention relates to an improved composite dental restorative material of the type including an inorganic glass component and an organic resin component.
Dental composite materials comprised of an inorganic component such as glass and an organic component such as a hardenable resin have been available since as early as 1965. Such materials are particularly valuable when used to restore the appearance and function of decayed, fractured, or otherwise defective or unsightly tooth surfaces. While prior art composite dental materials have proven to be adequate and beneficial for restoring teeth, they are often vulnerable to an undesirable amount of surface material loss under certain oral conditions. This has been referred to as loss of "anatomic form".
The reasons for loss of anatomic form have not been thoroughly established, though a hypothesis holds that there is a loss of surface filler particles due to inadequate or hydrolytically unstable adhesive bonding between the inorganic particle and the resin or polymer. This is then followed by wear of the polymer.
Individual filler particles are not worn away significantly when exposed on the surface since the bulk properties of most filler materials in use probably include adequate insolubility and resistance to wear. Thus, wear of filler particles is believed insignificant in comparison with the loss due to "plucking out" of weakly held particles because of forces resulting from a shearing of normal foods. A consequence of this hypothesis is that improved bonding between the filler materials and the organic polymeric matrix material is required.
The inventor previously disclosed a barium oxide-containing glass composition in "A New Series of X-Ray Opaque Reinforcing Fillers for Composite Materials", Journal of Dental Research, Volume 51, pages 177-182 (1972). The barium oxide composition disclosed in the referenced publication, however, produces a pH of about 9.3 when a 10% suspension of the composition is stirred in distilled water. This alkaline reaction may affect adversely the hydrolytic stability of the bonding between the organic polymer and the inorganic filler which results from the use of an organofunctional silane coupling agent interlayer.
To overcome such an adverse result and achieve other desirable features, continued experiments were conducted. Essential features of the filler for composite materials, where aesthetics of the composite restoration is important, include transparency and a refractive index in the vicinity of that of the composite resin polymer. This index of refraction is commonly about n.sub.D 1.55, although higher and lower values are sometimes encountered. Also, the inorganic filler component is preferably opaque to X-rays in order that subsequent evaluations and diagnostic studies, if necessary, may be performed by the dentist. The filler component should be nontoxic (except for certain industrial uses) and the thermal expansion coefficient should be minimized.
It was most fortunate that a kind of glass was discovered which, with proper preparation and treatment, seems capable of fulfilling all of the requirements of an inorganic component for a composite dental material comprised of the inorganic component and an organic resin or polymer component.